The present invention relates to a method of compression molding thermoplastic material.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
There are various proposals of compression molding thermoplastics. For example, published European Pat. Appl. No. 0 244 783 describes a method for molding articles by injecting melt into a pre-enlarged cavity that is formed between half-molds on respective platens. Compression of the injected melt begins before the injection process is over. Once the compression step has been completed and the cavity has been reduced to its final size, the melt is maintained under pressure until it solidifies.
Another approach is disclosed in published European Pat. Appl. No. 0 477 967 A2, which involves a closing of the half-molds until the mold cavity has a size less than the final cavity size. Melt is then injected into the cavity, accompanied by an opening of the mold so that the cavity size becomes larger than the final cavity. The mold then closes before or when the melt supply is completed.
Published European Pat. Appl. No. 0 868 988 A1 describes an injection molding machine with a vertical clamping unit to fully close a mold. Subsequently, the upper platen is set to a substantially pressure-free state and molten thermoplastic material is injected, so that the movable platen is pushed away by the melt pressure. After conclusion of the injection step, the gate is sealed and the compression step commences. Injection of melt into a horizontal clamping unit requires application of a counterpressure to prevent uneven melt distribution.
However, for a number of reasons, the various proposals are endowed with drawbacks and shortcomings relating for example to manufacturing techniques or to the effect that is hoped to be obtained but may not always be realized. For example, one drawback common to all prior art proposals is the requirement to precisely define the position of the half-molds before injection starts. In other words, the position of the platens must be precisely controlled. In particular when hydraulic injection molding machines are involved, this precise movement of the platens is very slow and time-consuming. Such a defined position cannot be realized when injecting melt and opening the movable platen through melt pressure. The compression step commences as soon as the entire amount of melt has been introduced and the gate has been sealed. Injection during compression becomes thus impossible. In particular, when a horizontal injection molding machine is involved, the absence of a predefined compression position causes uneven melt distribution.
It would therefore be desirable and advantageous to provide an improved method of compression molding thermoplastic material, to obviate prior art shortcomings and to produce high-quality products in a simple and yet at short cycle time, whereby a compression position is easy and quickly to realize.